An image forming apparatus of an electro photography system has a transfer device which transfers a toner image formed on the image carrier (for example, a photoreceptor, an intermediate transfer member, etc.) onto a sheet. There are some which have adopted a contact transfer system which contacts a transfer roller or a transfer belt onto a sheet as a transfer device for transferring a toner image onto the sheet.
FIG. 8 is an explanatory view showing a transfer area of a contact transfer system. The contact transfer system shown in FIG. 8 presses a transfer belt B to a photoreceptor K with a transfer roller T to form a transfer nip portion N between the photoreceptor K and the transfer belt B. In case when transferring a toner image formed on the photoreceptor K onto a sheet S which advances into the transfer nip portion N, voltage (transfer voltage) having a polarity which is opposite to the polarity of toner is applied to a transfer roller T to move the toner onto the sheet S from the photoreceptor K. For example, in case where the toner has a minus polarity, transfer voltage of a plus polarity is applied to the transfer roller T.
By the way, as shown in FIG. 8, in cases where the sheet S which has burrs X at the leading edge of the conveyance direction is advanced into the transfer nip portion N and the transfer voltage is applied to the transfer roller T, the direction of an electric field points toward the photoreceptor K from the transfer roller T. Therefore, electric discharge occurs in the opening which is formed at the leading edge of the sheet between the transfer belt B and the sheet S, and the edge of the sheet S in the conveyance direction is charged with the same polarity of the transfer voltage. As a result, the leading edge of the sheet S sticks to the photoreceptor K charged with an opposite polarity of the transfer voltage, and poor separation that the sheet S winds around the photoreceptor K occurs.
Then, technology for preventing poor separation of a sheet even in case where there is a burr at the edge of the sheet in the conveyance direction has been proposed.
The technology disclosed in Unexamined Japanese Patent Application Publication No. H10-240032 is a technology for providing the portion to which the transfer voltage is not applied in a predetermined area starling from the leading edge of the sheet in the conveyance direction. According to this technology, it is possible to prevent poor separation, because discharges do not occur even though there is a burr on the leading edge of the sheet in the conveyance direction and the sheet S does not wind around the image carrier (Unexamined Japanese Patent Application Publication No. H 10-240032 discloses a case in which the image carrier is an intermediate transfer drum).
According to the technology disclosed in Unexamined Japanese Patent Application Publication No. H10-240032, the applied voltage in the predetermined area of the leading edge of a sheet in the conveyance direction is 0V (or the voltage value is close to 0V), and although the sheet has a burr, discharge seldom occurs. However, the polarity of the leading edge of the sheet in the conveyance direction does not become the polarity which repels with an image carrier. Therefor; as shown in FIG. 8, when there is a gap between the transfer belt B and the leading edge of sheet S, the leading edge of sheet S will be hard to be adsorbed by the transfer belt B but the leading edge of the sheet S will be easily stacked to the photoreceptor K. Therefore, according to the technology disclosed in Unexamined Japanese Patent Application Publication No. H10-240032, the poor separation cannot be fully prevented.
An object of the present invention is to provide a transfer device and an image forming apparatus which are capable of preventing poor separation and fully secure the density of a toner image to be transferred onto a sheet.